1. Field
The present application relates to a lighting apparatus including a translucent composite of garnet-based emissive layers.
2. Description
Solid state light emitting devices such as light emitting diode (LED), organic light emitting diode (OLED) or sometimes called organic electroluminescent device (OEL), and inorganic electroluminescent device (IEL) have been widely utilized for various applications such as flat panel display, indicator for various instrument, signboard, and ornamental illumination, etc. As the emission efficiency of these light emitting devices continues to improve, applications that require much higher luminance intensity, such as automobile headlights and general lighting, may soon become feasible. For these applications, white-LED is one of the promising candidates and have attracted much attention.
Conventional white-LED have been manufactured based on the combination of blue-LED and yellow light-emitting YAG phosphor powder dispersed in plastic encapsulant resin like epoxy and silicone. However, since the particle size of YAG phosphor powder utilized for this system is around 1-10 μm, the YAG powders dispersed in the encapsulant resin medium can cause strong light scattering. As a result, a considerable portion of both the incident light from the blue LED and the yellow emitting light from YAG powders ends up being back scattered and dissipated as a loss of white light emission.
One approach to solve this problem is to use one or more phosphor ceramic plates or laminated films that are positioned to receive emissions from blue emitting light emitting diodes instead of phosphor powders suspended in a transparent matrix. With optimized sintering conditions, combined with the use of phosphors in nanoparticle form, it is possible to obtain a translucent or transparent ceramic. Since an LED with translucent/transparent phosphor ceramics has less backscattering loss while maintaining, or in some cases, increasing the luminance efficiency of the phosphor layer, it may emit stronger white light than conventional YAG powder/resin system. In addition, since a ceramic plate/film is more stable against heat and light irradiation even under UV, temperature fluctuation of operational property, durability, and lifetime of the LED can be improved.
In order to achieve a lower correlated color temperature (CCT) and higher color rendering index (CRI) in a YAG: Ce LED system, a red emitting component is needed. U.S. Patent Publication No. 2007/0215890, which is hereby incorporated by reference in its entirety, discloses the use of a red phosphor platelet and a green phosphor platelet in combination with a blue LED to create a white light LED. U.S. Pat. No. 7,446,343, which is hereby incorporated by reference in its entirety, uses yellow and red phosphor layers in LEDs, where the red phosphor candidates need to have Eu3+ as dopant. Due to the use of different dopants for the yellow and the red layers, they have to be prepared separately. Furthermore, since Eu3+-doped compounds have a primary absorption region in the UV spectra instead of the blue visible light, some have resorted to using nitride or sulfide phosphors that can be activated by blue light. For example, U.S. Pat. No. 7,361,938, which is hereby incorporated by reference in its entirety, discloses the use of a yellow emitting garnet (YAG: Ce) phosphor and a red emitting nitride or sulphide phosphor. However, these nitride or sulfide emissive elements are not desirable due to the processing difficulties and/or chemical/thermal stability concerns. Thus there is a need for a white light LED device that utilizes blue light and green/yellow/red phosphor layers and offers simplified processing.
Furthermore, some lighting systems, particularly those for daily and landscape uses, as well as high power lighting systems for industrial uses, have a somewhat lower color temperature, T≦3500K, falling into the category of warm white lighting. If the color temperature is higher, T>4600K, the lighting system will be categorized as cold white light. For solid-state lighting purposes, warm white light is especially preferred because it has the same color temperature as standard incandescent bulbs. Thus, there is also a need for warm white light LED devices that utilize blue light and green/yellow/red phosphor layers, as well as simple and economical processes for making these LED devices.